Transparent insulation has long been known and can comprise, in accordance with conventional practices, so-called double windows or multiple-pane window structures, as well as walls formed by glass blocks. These ligh-permeable thermally insulating structures can be used to supply daylight to workrooms and living quarters, but prevent the loss of heat therefrom. They have been found to have greater heat-lagging effectiveness than simple windows with single glass panes interposed between the external environment and the interior of the structure.
In recent years multiple-pane wall structures and windows have been provided for this purpose and, in addition, for the covering of planar solar-energy collectors so that the radiant energy from the sun can penetrate to the heat-absorbing surface but loss of heat by convection, reradiation and contact or conduction from this surface is precluded.
Multiple-pane transparent thermally insulating walls are satisfactory only as long as the differential between the temperature at the pane and the highest ambient temperature is relatively low. This is usually the case when the thermally insulating transparent structure forms part of a glass door or window in dwellings and work places.
However, when such a multiple-pane wall is used in conjunction with a solar-energy collector, numerous disadvantages are found to arise which have limited the desirability of such structures.
Thus, the pane of the glass wall closest to the energy-collecting surface is generally at an extremely high temperature which can be about 150.degree. C. or more. As a result, this glass pane or the entire assembly of glass panes must be of special temperature-resistant or refractory glass. Fabricating the covering for a solar-energy collector from such special glasses has been found to be inordinately costly.
Generally, moreover, the outer glass pane is disposed in the collector not vertically but rather with an inclination to the vertical and hence is subject to high bending stresses because of the weight of the glass pane, snow loads, hail and other environmental effects. As has been learned from greenhouse construction, such stresses can only be taken up reliably by glass panes whose thickness is several millimeters, thereby increasing the weight of the glass pane and providing a greater thickness of glass which must be traversed by the solar energy. This has been found to increase the cost of solar-energy collectors and reduce the efficiency thereof. To overcome these disadvantages at least in part, it has been proposed to substitute for the silicate-glass panes foils or plates of a transparent or light-permeable synthetic resin. However, while all synthetic resins which can be used for this purpose have a substantially lower specific gravity than silicate glass and a high impact-bending strength, it has been found that such foils and plates, apart from those made from expensive glass-fiber-reinforced synthetic resins, have poor shape retentivity or stability at high temperatures and hence a low resistance to long-term exposure to elevated temperatures. Thus it is not possible or practical to use the synthetic resin directly adjacent the collector surface. Furthermore, the light transmissivity of such synthetic-resin foils is lower than that of glass.
There have already been suggestions for the covering of the absorber surface of a solar collector with glass blocks or the like. It has been the practice to design the glass blocks as half shells which are pressed from glass and are assembled into the closed hollow bodies. Because of the pressing process, however, these structural elements of glass have a great wall thickness and are relatively heavy. This again increases the size of the support structure which must be provided to carry the transparent thermal barrier. In addition, the light permeability or transmissivity of thick-walled glass elements is limited and such block have been found unsatisfactory for solar-energy collectors.